1. Field of the Invention
The invention relates to an apparatus and method for recovering solids from a polymer-diluent slurry withdrawn from an olefin polymerization reactor.
2. Description of the Prior Art
Polyolefins, for example polyethylene and polypropylene, are commercially produced in different types of reactors including solution, slurry, stirred-bed and gas phase systems. In slurry systems, an inert diluent liquid circulates within a continuous pipe loop which may be oriented vertically or horizontally. The olefin and catalyst are fed to the reactor and the resulting solid polymer forms a slurry. Because the accumulating product must be withdrawn while still maintaining the high pressure and temperature in the reactor, engineers have continued to seek improvements in polymer removal from a slurry reactor.
U.S. Pat. No. 3,242,150 to Scoggin teaches a conventional means for removing solid polymer from a vertical loop slurry reactor. Solid polymer accumulates in a small-diameter tube or settling leg extending from the lower side of the loop reactor. The polymer then exits through a valve which periodically opens fully and closes. Scoggin also discloses a two-valve arrangement by which a bottom valve is closed and an upper valve opens to trap polymer product therein, then the upper valve is closed and the bottom valve opens to allow the polymer to escape toward the product recovery zone.
U.S. Pat. No. 3,816,383 to Stotko describes a process for separating the solid olefin polymer from liquid diluent by passing the slurry through a cyclone separator, removing an overflow stream from the separator concentrated in the liquid diluent, removing an underflow stream from the separator concentrated in the solid polymer, combining the overflow with the residual underflow, and recycling the resulting mixture to the reactor, thus avoiding blockage in the separation system.
U.S. Pat. No. 3,639,374 to Stryker, Jr., teaches a polymerization system by which a catalyst enters a reactor containing liquid propylene to form a polymer slurry within the reactor. The slurry then exits to a cyclone separator at a percent solid content of approximately 50-60 percent whereby the propylene diluent flashes from the polymer product.
Separation of the polymer from the accompanying liquids and gases can be accomplished in single or multiple-stage processes. U.S. Pat. No. 5,575,979 and its divisional U.S. Pat. No. 5,597,892 to Hanson describe a method for recovering solids from a polymer-diluent slurry in a two-stage method. The diluent vaporizes from the polymer slurry in a first cyclonic flash vessel, then exits to be condensed and recycled to the reactor. The polymer solids pass into an extended solids reservoir of the cyclonic flash vessel until it is at least partially full and thereafter pass to a non-cyclonic second flash vessel to vaporize residual diluent.
In one aspect the invention relates to a method of continuously removing polymer from a pressurized loop polymerization reactor containing a slurry of polymer particles and liquids, and an apparatus suitable for this method. A slurry of polymer particles is continuously conveyed from polymer-rich zones of the reactor through a discharge means and a transfer line. The slurry enters a non-cyclonic flash vessel maintained at a lower pressure, whereupon the particles separate and settle to the bottom of the flash vessel. In one embodiment, more than one flash vessel is employed. The flash vessel has vertical sidewalls and a conical bottom in which a minimum level of polymer is maintained for a dynamic seal.